The present invention relates to an electronic vehicular anti-theft system which is as simple as a conventional ignition key to use but can effectively protect vehicles from theft.
Various anti-theft systems have been proposed and many of them combine a mechanical key and an electric or electronic key. This is advantageous because the users can use them much in the same way as conventional keys and can yet get the benefits of electronic keys which can not be easily bypassed or duplicated by unauthorized persons. For instance, U.S. Pat. No. 3,515,891 teaches a key incorporating a crystal element and U.S. Pat. No. 4,148,372 discloses a key incorporating a resistor pellet. This crystal element or the resistor pellet is connected to a circuit mounted on the vehicle when the key is fitted into the key hole and is determined to be a correct one or not by this circuit. However, these keys must be incorporated with various fixed elements having unique characteristics which cannot be changed easily. Therefore, the production of such keys necessarily tends to be complicated and authorized duplication of the keys is cumbersome.
U.S. Pat. No. 4,222,034 teaches a combination of a mechanical key and an ultrasonic transmitter and Japanese patent laid-open publication No. 57-90237 discloses a combination of a key and a radio transmitter. These electronic keys may be provided with a large number of code combinations which could be "written" or "burnt" into the keys but the receivers of ultrasonic sound and radio waves are relatively complicated and could be affected by external interferences.